Week 1, Lecture 2

Cellular Form and Function

Reference Book:

Anatomy & PhysiologyThe Unity Of Form And Function

Ninth Edition;Kenneth S. Saladin

Lecture delivered by

Dr Vini Gautam

Lecturer

Department of Biomedical Engineering

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Lecture Outline

• Cells, shapes and sizes
• Basic components of a cell
• Plasma membrane

Quick Quiz

• Membrane transport

Quick Quiz

• Cytoskeleton
• Some organelles
• Inclusions
• Nucleus and genetics

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Introduction

• All organisms are composed ofcells
• Simplest structural and functional unit of life
• Cells of all species exhibit biochemical similarities
• For humans, cells are important for understanding
• How does the body work?
• What is the mechanism of a disease?
• How do treatments and therapy work?

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Common Cell Shapes

About 200 types of cells in human body with varied shapes

Note: A cell’s shape can appear different if viewed in a different type of section (longitudinal versus cross section)

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Cell Sizes

Human cell sizes

• Most cells about 10 to 15μm in diameter
• Egg cells (very large) 100μm diameter

• Limit on cell size: an overly large cell cannot support itself,may rupture
• For a given increase in diameter, volume increases more thansurface area
• Volume proportional to cube of diameter
• Surface area proportional to square of diameter

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The Relationship Between Cell Surface Area and Volume

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a: © AlvinTelser/McGraw-Hill Education;b: ©BiophotoAssociates/Science Source

How does a cell look like under a microscope?

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Basic Components of a Cell

1. Plasma (cell) membrane

• Surrounds cell, definesboundaries
• Made of proteins and lipids

2. Cytoplasm

• Organelles
• Cytoskeleton
• Inclusions (stored or foreignparticles)
• Cytosol (intracellular fluid,ICF)

Extracellular fluid (ECF)

• Fluid outside of cells includestissue (interstitial) fluid

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1. The Plasma Membrane

Plasma membrane = border of the cell

• Appears as pair of dark parallel lines when viewed withelectron microscope
• Hasintracellularandextracellularfaces

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The Plasma Membrane

Access the text alternative for slide images.

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Plasma membrane component #1 : Lipids (98%)

Phospholipids

• 75% of membrane lipids arephospholipids
• Amphipathic molecules arranged in abilayer
• Hydrophilic phosphate headsface wateron each side of membrane
• Hydrophobic tails—are directed towardthe center, avoiding water
• Drift laterally, keeping membrane fluid

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Membrane Lipids

Cholesterol

• 20% of the membrane lipids
• Holds phospholipids still and can stiffen membrane

Glycolipids

• 5% of the membrane lipids
• Phospholipids with short carbohydrate chains onextracellular face
• Contributes to glycocalyx—carbohydrate coating on cellsurface

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Membrane proteins

• 2% of the molecules but 50% of the weight of membrane

Integral proteins—penetrate membrane

• Trans-membraneproteins = pass completely through
• Hydrophilic regions contact cytoplasm, extracellular fluid
• Hydrophobic regions pass through lipid of the membrane
• Some drift in membrane; others are anchored tocytoskeleton

Peripheral proteins

• Adhere to one face of the membrane
• Usually tethered to the cytoskeleton

Plasma membrane component #2 : Proteins (98%)

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Transmembrane Proteins

Figure 3.6

Access the text alternative for slide images.

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Functions of different Membrane Proteins

Access the text alternative for slide images.

To ‘stick’ cells inplace

What kind of cellam I?

Involved in the action of drugs

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A Second-Messenger System

Figure 3.8

Access the text alternative for slide images.

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Functions of different Membrane Proteins

Involve TRANSPORT across the plasma membrane

Crucial to nerve and muscle function

© McGraw Hill

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Break (2 min)

See poll for a quick quiz and answer

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Functions of different Membrane Proteins

Involve TRANSPORT across the plasma membrane

Crucial to nerve and muscle function

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Membrane Transport

Plasma membrane (and organelle membranes) areselectively permeable—allowing some things through, butpreventing others from passing

Passivetransport requireno energy(ATP)

• Random molecular motion of particles provides necessaryenergy
• Filtration, diffusion, osmosis

Activetransportconsume energy(ATP)

• Vesicular transport

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Passive transport method #1:Filtration

Filtration—particles are driven through membrane byphysicalpressure

Examples

• Filtration of water and small solutes through gaps incapillary walls
• Allows delivery of water and nutrients to tissues
• Allows removal of waste from capillaries in kidneys

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Filtration Through the Wall of a Blood Capillary

Figure 3.13

Access the text alternative for slide images.

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Passive transport method #2:Diffusion

Simple diffusion—net movement of particles from place ofhigh concentration to place of lower concentration

• Due to constant, spontaneous molecular motion
• Molecules collide and bounce off each other

Substances diffuse down theirconcentration gradient

• Does not require a membrane
• Substance can diffuse through a membrane if themembrane is permeable to the substance

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What can affect Diffusion?

Factors affectingdiffusion ratethrough a membrane

• Temperature: ↑temp., ↑motion of particles
• Molecular weight: larger molecules move slower
• Steepness of concentrated gradient: ↑difference,↑rate
• Membrane surface area:↑area,↑rate
• Membrane permeability:↑permeability,↑rate

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Passive transport method #3:Osmosis

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Passive transport method #3:Osmosis

Osmosis—netflow of waterthrough a selectively permeablemembrane

• Water moves from the side where it (water) is moreconcentrated to the side where it is less concentrated
• Solute particles that cannot pass through the membrane “draw”water from the other side

Crucial consideration for IV fluids

Osmotic imbalances underlie diarrhea, constipation, edema

Cool fact:While water can diffuse through phospholipid bilayers,osmosis is enhanced by aquaporins (channel proteins in membranespecialized for water passage)

• Cells can speed osmosis if needed by installing moreaquaporins

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Tonicity

Tonicity—ability of a surrounding solution (bath) to affect fluid volume andpressure in a cell

• Depends on concentration of nonpermeating solutes

Hypotonic solution—causes cell to absorb water and swell

• Has a lower concentration of nonpermeating solutes than intracellularfluid (ICF)
• Distilled water is an extreme example

Hypertonic solution—causes cell to lose water and shrivel (crenate)

• Has a higher concentration of nonpermeating solutes thanICF

Isotonic solution—causes no change in cell volume

• Concentrations of nonpermeating solutes in bath andICFare thesame
• Normal saline (0.9% NaCl) is an example

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Effects of Tonicity on RBCs

(a-c): © David M. Philips/Science Source

Swollen cell

Crenated cell

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Osmolarity

One osmole (osm) = 1 mole of dissolved particles

• Takes into account whether solute ionizes in water
• 1 M glucose is 1osm/L
• 1 M NaCl is 2osm/L

Osmolarity = number of osmoles per liter of solution

• Body fluids contain a mix of many chemicals, andosmolarity is the total osmotic concentration of all solutes
• Blood plasma, tissue fluid, and intracellular fluid are 300milliosmoles per liter (mOsm/L)

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Carrier-Mediated Transport

Transport proteins in membrane carry solutes into or out ofcell (or organelle)

Specificity

• Transport proteins are specific for particular solutes
• Solute (ligand) binds to receptor site on carrier protein
• Solute is released unchanged on other side of membrane

Saturation

• As solute concentration rises, the rate of transport rises,but only to a point—transport maximum (Tm)
• Transport maximum = transport rate at which all carriersare occupied

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Carrier-Mediated Transport2

Three kinds of carriers

• Uniport—carries one type of solute
• Example: calcium pump
• Symport—carries two or more solutes simultaneously insame direction (cotransport)
• Example: sodium–glucose transporters
• Antiport—Carries two or more solutes in oppositedirections (countertransport)
• Example: sodium–potassium pump removes Na+, brings in K+

Three mechanisms of carrier-mediated transport

• Facilitated diffusion, primary active transport, secondaryactive transport

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1. Facilitated Diffusion

Facilitated diffusion: a carrier moves solute down itsconcentration gradient

Does not consume ATP

Solute attaches tobinding siteon the carrier, carrier changesconformation, then releases solute on other side ofmembrane

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Facilitated Diffusion

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2. Primary active transport across a carrier protein

• Primary active transport: the carrier protein moves solutethrough a membraneagainst it’s concentration gradient
• The carrier protein uses ATP for energy

Examples:

• Sodium–potassium pump
• Moves ions from low concentration to higher concentration
• Hard work = used ATP

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• A nerve cellat rest:
• more Na outside thecell
• more K inside the cell
• Potential differenceacross the membrane

= resting potential

The sodium–potassium pump (Na+− K+pump)

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The Sodium–Potassium Pump (Na+–K+)

Each Na-K pump cycle:

• consumes one ATP
• Pushes 3 Na+out

Takes 2 K+in

-Moves ions fromlow concentration tohigher concentration

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Na+− K+pump functions

• Keeps K+concentration higher and Na+concentrationlower within the cell
• Maintains negatively charged resting membranepotential
• Resets the cell to original electrochemical gradient
• Necessary because Na+and K+constantly leak throughmembrane
• Up to half of daily calories utilized forNa+− K+pump
• Regulates solute concentration and thus osmosis andthus cell volume

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3. Secondary active transport

Secondary active transport

• Carrier protein moves solutethrough membrane but only usesATP indirectly
• Example: sodium–glucosetransporter (SGLT)
• Moves glucose into cell whilesimultaneously carrying sodium downits gradient
• Depends on the primary transportperformed byNa+− K+pump
• Does not itself use ATP

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Functions of different Membrane Proteins

Involve TRANSPORT across the plasma membrane

Crucial to nerve and muscle function

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Vesicular Transport

Vesicular transport—moves large particles, fluid droplets, ornumerous molecules at once through the membrane

Vesicles = bubble-like enclosures of membrane; utilizes motorproteins energized by ATP

Endocytosis—vesicular processes that bring material into cell

• Phagocytosis—“cell eating,” engulfing large particles
• Pinocytosis—“cell drinking,” taking in droplets of ECF containingmolecules useful in the cell
• Receptor-mediated endocytosis—particles bind to specificreceptors on plasma membrane

Exocytosis—discharging material from the cell

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Endocytosis and exocytosis

© McGraw Hill

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Break (2 min)

See poll for a quick quiz and answer

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The cell interior

Note: A complete summary of cellular components can be found inTable 3.4in the reference book

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1. The Cytoskeleton

Cytoskeleton—network of protein filaments and cylinders

• Determines cell shape, supports structure, organizes cellcontents, directs movement of materials within cell,contributes to movements of the cell as a whole

Composed of: microfilaments, intermediate fibers,microtubules

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The Cytoskeleton

Microfilaments

• 6 nm thick; made of actinprotein
• Form terminal web

Intermediate filaments

• 8 to 10 nm thick; made ofprotein keratin
• Give cell shape, resist stress

Microtubules

• 25 nm thick; consists of proteintubulin; can come and go
• Maintain cell shape, holdorganelles, act as railroadtracks for walking motorproteins, form mitotic spindle

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2. Inclusions (are not cell organelles)

Two kinds of inclusions

• Stored cellular products
• Glycogen granules,pigments, and fat droplets
• Foreign bodies
• Viruses, intracellularbacteria, dust particles, andother debris phagocytized bya cell

Never enclosed in a unitmembrane

Not essential for cell survival

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3. Cell Organelles

Internal structures of a cell,carry out specialized metabolictasks

Membranous organelles

• Nucleus, mitochondria,lysosomes, peroxisomes,endoplasmic reticulum, andGolgi complex

Non-membranous organelles

• Ribosomes, centrosomes,centrioles, basal bodies

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Endoplasmic Reticulum

• System of channels (cisterns) enclosed by membrane
• Produces phospholipids, steroids and proteins
• Detoxifies alcohol and other drugs
• Calcium storage

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Golgi Complex

Golgi complex—a system of cisternsthat synthesizes carbohydrates andputs finishing touches on proteinsynthesis

• Receives newly synthesizedproteins from rough ER
• Sorts proteins, splices some, addscarbohydrate moieties to some,and packages them intomembrane-bound Golgi vesicles
• Some vesicles become lysosomes
• Some vesicles migrate to plasmamembrane and fuse to it
• Some become secretory vesiclesthat store a protein product for laterrelease

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Lysosomes

Lysosomes—package of enzymes bound by a membrane

• Generally round, but variable in shape

Functions

• Intracellular hydrolytic digestion of proteins, nucleic acids,complex carbohydrates, phospholipids, and othersubstances
• Autophagy—digestion of cell’s surplus organelles
• Autolysis—“cell suicide”: digestion of a surplus cell byitself

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Mitochondria

• Organelles specialized forsynthesizing ATP,hencecalled “Powerhouses” of thecell
• Matrix contains ribosomes,enzymes used for ATP synthesis,small circular DNA molecule
• Multiple molecules ofmitochondrial DNA (mtDNA)
• Energy is extracted fromorganic molecules andtransferred to ATP

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The Nucleus

Nucleus—usually largest organelle (~ 5μmin diameter)

Nuclear envelope: perforated by nuclearpores formed by rings of proteins

• Regulate molecular traffic throughenvelope
• Helps regulate cell life cycle

Nucleoplasm—material in nucleus

• Chromatin (thread-like) composed ofDNA and proteins (genetics)

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Topics covered today

Cells, shapes and sizes

Basic components of a cell

Plasma membrane

Membrane transport

Cytoskeleton

Organelles

Inclusions

Nucleus

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Topics to be covered in the next lecture

Kinds of tissues in human body:

Epithelial tissue

Connective tissue

Nervous and Muscular (excitable) tissue

Tissue growth

Tissue development

Stem cells

Tissue repair and degeneration

Stem cell therapy, regenerative medicine, tissue engineering